1. Field of the Invention
This invention relates to a radiation image read-out apparatus for use in a radiation image recording and reproducing system.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinbelow referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object to have a radiation image stored therein, and is then scanned with stimulating rays such as a laser beam which cause it to emit light in the pattern of the stored image. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image on a recording medium such as a photographic light-sensitive material or on a display device such as a cathode ray tube (CRT).
FIG. 2 is a perspective view showing the conventional read-out apparatus used in the aforesaid radiation image recording and reproducing system for scanning a stimulable phosphor sheet carrying a radiation image stored therein by stimulating rays such as a laser beam which cause the sheet to emit light in proportion to the stored radiation energy, and photoelectrically detecting the emitted light.
In the apparatus of FIG. 2, stimulating rays 2 are emitted by a stimulating ray source 1, and the beam diameter of the stimulating rays 2 is strictly adjusted by a beam expander 3. The stimulating rays 2 are then deflected by a light deflector 4 formed of a galvanometer mirror or the like, and are made to impinge upon the stimulable phosphor sheet 10 by a plane reflection mirror 5. Between the light deflector 4 and the plane reflection mirror 5 is positioned an f.theta. lens 6 for maintaining the beam diameter of the stimulating rays 2 uniform during the equal-speed scanning of the stimulating rays 2 on the stimulable phosphor sheet 10. While the stimulating rays 2 impinge upon the stimulable phosphor sheet 10, the sheet 10 is moved in the direction as indicated by the arrow A (i.e. sub-scanning direction) and, consequently, the whole area of the sheet 10 is exposed to and scanned by the stimulating rays 2. Upon exposure to the stimulating rays 2, the stimulable phosphor sheet 10 emits light in proportion to the radiation energy stored therein, and the light emitted enters a light guide member 8. The light guide member 8 has a linear light input face 8a positioned close to the scanning line on the stimulable phosphor sheet 10, and a ring-shaped light output face 8b in close contact with the light receiving face of a photodetector 9, which may be a photomultiplier. The light guide member 8 and the photodetector 9 constitute a photoelectric read-out means 7. The light guide member 8 is fabricated of a transparent thermoplastic resin sheet such as an acrylic resin sheet so that the light entering from the light input face 8a can be transmitted to the light output face 8b by total reflection inside of the light guide member 8. The light emitted by the stimulable phosphor sheet 10 upon stimulation thereof is guided inside of the light guide member 8, emitted from the light output face 8b of the light guide member 8 and received by the photodetector 9. The light guide member 8 may be of a shape and a material as disclosed in U.S. Pat. No. 4,346,295.
The light receiving face of the photodetector 9 is provided with a filter (not shown) for transmitting only the light having the wavelength distribution of the light emitted by the stimulable phosphor sheet 10 and cutting off the light having the wavelength distribution of the stimulating rays 2, so that the photodetector 9 can detect only the light emitted by the stimulable phosphor sheet 10 upon stimulation thereof. The light detected by the photodetector 9 is converted into an electric signal, amplified to an appropriate level by an amplifier 11 the sensitivity of which has been adjusted by an amplification degree setting value (a), and then sent to an A/D converter 12. In the A/D converter 12, the electric signal is converted into a digital signal by use of a scale factor which has been set by a scale factor setting value (b) adjusted in advance in accordance with the image pattern of an object to suit the width of signal fluctuation. The digital signal thus obtained is sent to a signal processing circuit 13, in which it is processed to obtain a radiation image suitable for viewing, particularly for diagnostic purposes. The electric image signal obtained by the signal processing circuit 13 is then used to reproduce a visible image on a recording material such as a photographic film or a display device such as a CRT.
In general, the radiation image read-out apparatus is provided with a reflection mirror 14 for obtaining an image of high quality by improving the light guiding efficiency for the light emitted by the stimulable phosphor sheet 10 when it is exposed to the stimulating rays 2. The reflection mirror 14 is positioned on the side opposite to the light guide member 8 with respect to the scanning line of the stimulating rays 2 on the stimulable phosphor sheet 10 so as to reflect the light, which is emitted by the sheet 10 in the direction reverse to the light guide member 8, towards the light guide member 8.
FIG. 3 is a schematic side view taken in the direction as indicated by the arrow B of FIG. 2 and showing the condition of reflection of stimulating rays. In general, in the image read-out as described above, stimulating rays 2a (hereinafter referred to as the scanning stimulating rays) impinging upon the stimulable phosphor sheet 10 for scanning it are reflected by the sheet 10. Reflected stimulating rays 2b are again reflected by various components (the reflection mirror 14, the light input face 8a of the light guide member 8, etc.) of the read-out apparatus onto portions of the sheet 10 that have not yet been scanned, thereby stimulating the non-scanned portions and causing them to emit light (this phenomenon is hereinafter referred to as the flare phenomenon). When the flare phenomenon arises, since the reflected stimulating rays 2b again impinge upon the non-scanned portions of the sheet 10 outside of the picture element detected at a given instant by scanning with the scanning stimulating rays 2a, light emitted by the non-scanned portions is detected by the photoelectric read-out means 7 together with the light emitted by the scanned picture element of the sheet 10. Therefore, the reproduced image thus obtained becomes incorrect, and contrast of the image becomes low.
In the radiation image read-out apparatus provided with the reflection mirror 14, the reflection mirror 14 constitutes a main cause of the flare phenomenon. This is because, though the reflection mirror 14 improves the light guiding efficiency by reflecting the light emitted by the stimulable phosphor sheet 10 towards the light guide member 8, it also reflects the scanning stimulating rays 2a. Therefore, the stimulating rays 2b reflected by the sheet 10 are further reflected by the reflection mirror 14 onto the non-scanned portions of the sheet 10 outside of the scanned picture element, or are reflected thereby onto the light input face 8a of the light guide member 8 and then reflected by the light input face 8a onto the non-scanned portions of the sheet 10.